1. Field of the Invention
This invention relates to precipitation hardening stainless steels which are superior in cold workability, and excellent in proof stress in a state being subjected to aging treatment at a temperature higher than peak aging temperature.
2. Description of the Prior Art
As a material for bolts and shafts of various kinds, a precipitation hardening stainless steel has been used, which is specified in JIS G 4303 as SUS 630 corresponding to ASTM 630 and excellent in corrosion resistance and strength.
In a case of manufacturing bolts or so from the conventional SUS 630 steel, the steel is subjected to hot forging. Although the hot forging is easy to work the steel material, the number of steps in the manufacturing process becomes larger since it is required for machinery cuts after that, and there is a problem in the cost.
Therefore, it is desirable to form the steel material into the bolts or so through cold forging. However, SUS 630 steel is hard as much as HRC 35 in a solution treated state (ST-state), and is inferior in cold workability.
Accordingly, as a method for improving the cold workability of SUS 630 steel, reduction of C and N content in the steel has been being studied. Namely, this is a conception to lower the hardness of the parent phase (martensite phase) in the ST-state as compared with that of the conventional SUS 630 steel by decreasing the C and N content in the steel in order to improve the cold workability.
In general, the precipitation hardening stainless steel SUS 630 is so designed as to be brought with high strength by subjecting the steel to aging treatment after the solution treatment and depositing the precipitation hardening phase from the parent phase.
In this case, it is possible to obtain the maximum strength by carrying out peak aging treatment (aging treatment at 480.degree. C.), however there is a problem in that toughness of the steel is lowered by the peak aging treatment. Accordingly, in a case where the toughness is necessary, such a method to ensure the required toughness by subjecting the steel to overaging treatment at a temperature higher than the peak aging temperature (at 620.degree. C., for example) is carried out.
On the other side, there is a problem in that austenite (.gamma.-phase) is precipitated from the parent phase owing to reverse transformation in the case of subjecting the steel to the overaging treatment at a high temperature like this. Namely, if the reverse-transformed austenite of this kind is precipitated, deterioration of the proof stress after the aging treatment becomes remarkable, the strength of the parent phase in the overaging treated state becomes lower than that of the conventional SUS 630 steel especially in the case of the precipitation hardening stainless steel of which the total sum of C and N content is lowered, and it has became clear that there are cases where the proof stress of the steel becomes lower than the value specified in JIS G 4303 owing to the precipitation of the .gamma.-phase.